1. Field of the Invention
The present invention relates to a variable capacitor capable of changing static capacitance.
2. Description of the Related Art
In the technical field of wireless communication equipments such as a mobile phone, the increase in number of components to be incorporated in the equipment for achieving higher performance has been creating growing demand for smaller sizes of high-frequency and RF circuits. In order to satisfy such demand, a technique called micro-electromechanical systems (hereinafter, MEMS) has been employed for micronization of various components constituting the circuit, including a variable capacitor. The variable capacitor is an essential component in a variable-frequency transmitter, a tuned amplifier, an impedance matching circuit and so on. The variable capacitor that can be obtained based on the MEMS technique can be found, for example, in the patent documents 1 and 2 cited here below.
Patent document 1: JP-A-2004-6588
Patent document 2: JP-A-2004-127973
The variable capacitor manufactured through the MEMS technique generally includes a fixed electrode and a movable electrode disposed so as to oppose each other. The movable electrode is set to move closer to and away from the fixed electrode. In such variable capacitor, a voltage applied between the fixed electrode and the movable electrode is controlled so as to adjust static attraction generated therebetween, to thereby adjust the position of the movable electrode, hence the gap between the electrodes, thus varying the static capacitance.
Also, in the variable capacitor the fixed electrode may be provided with a dielectric film on its surface opposing the movable electrode, to prevent a direct contact with the movable electrode, which is set to move toward the fixed electrode, because the direct contact of the electrodes causes a short-circuit therebetween. For controlling the variable static capacitance, some variable capacitors are designed such that the movable electrode can be intentionally brought into contact with the dielectric film on the surface of the fixed electrode, while in other variable capacitors the movable electrode may accidentally contact the dielectric film on the surface of the static capacitance, during the controlling process of the static capacitance.
When the movable electrode and the dielectric film on the surface of the fixed electrode come into contact, with the voltage being applied between the both electrodes, the charge may transfer between the movable electrode and the dielectric film. To be more detailed, when the movable electrode and the dielectric film on the surface of the fixed electrode come into contact, with the voltage being applied between the fixed electrode and the movable electrode so as to grant a positive potential to the movable electrode, an electron (negative charge) may transfer from the dielectric film to the movable electrode via the interface between the movable electrode and the dielectric film, to thereby create a hole (positive potential) in the dielectric film. When the movable electrode and the dielectric film on the surface of the fixed electrode come into contact, with the voltage being applied between the fixed electrode and the movable electrode so as to grant a positive potential to the fixed electrode, an electron (negative charge) may transfer from the movable electrode to the dielectric film via the interface between the movable electrode and the dielectric film. Accordingly, repeated contacts between the movable electrode and the dielectric film on the surface of the fixed electrode often cause the dielectric film to be significantly charged, because of such charge transfer.
Although the voltage applied between the movable electrode and the fixed electrode remains the same, the net magnitude of the field created between the electrodes of the variable capacitor fluctuates depending on whether and how much the dielectric film is charged, and hence a minimum driving voltage, to be applied between the electrodes so as to cause the movable electrode to move from its initial position, is not constant. Besides, the relationship between the static capacitance and the driving voltage (voltage to be applied between the electrodes to attain a predetermined static capacitance, or the gap between the electrodes) for driving the variable capacitor, or the movable electrode thereof, also fluctuates depending on whether and how much the dielectric film is charged. Thus, the driving voltage characteristic fluctuates depending on whether and how much the dielectric film is charged. Although the movable electrode is provided with the dielectric film on its surface, instead of or in addition to the dielectric film on the surface of the fixed electrode, the driving voltage characteristic may likewise fluctuate depending on whether and how much the dielectric film is charged. In the variable capacitor of the conventional design to adjust the static capacitance by controlling the gap between the electrodes, the amplitude of the fluctuation in driving voltage characteristic is significant. In addition, a larger variation ratio is generally required, with respect to the static capacitance of the variable capacitor.